Method of driving liquid crystal display

ABSTRACT

A method of driving a liquid crystal display device during one display frame includes the steps of applying one of a high-level common voltage and a low-level common voltage to a plurality of liquid crystal cells of the liquid crystal display device to write data into the liquid crystal cells within a time interval shorter than one display frame interval, and turning on a backlight after said data writing to display an image.

[0001] The present invention claims the benefit of Korean PatentApplication No. P2000-79988 filed on Dec. 22, 2000 in Korea, which ishereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a technique for driving a liquidcrystal display, and more particularly to a method of driving a liquidcrystal display that is capable of preventing a generation of a residualimage and a flicker phenomenon to improve a picture quality.

[0004] 2. Description of the Related Art

[0005] Generally, a liquid crystal display (LCD) of an active matrixdriving system uses thin film transistors (TFT's) as switching devicesto display a natural moving picture. Since such a LCD can be implementedinto a device smaller in size than the presently existing Brown tube, ithas been widely used as a monitor for personal and notebook computers aswell as office automation equipment such as copy machines and portableequipment such as cellular phones and pagers.

[0006] In FIG. 1, a conventional LCD allows a liquid crystal displaydevice to record image data on the liquid crystal cells during a displayframe of 16.67 ms, thereby displaying an image, and continuously recordsimage data in each display frame. In such a LCD device, a backlight isalways turned on. In the course of recording the data in any one displayframe, a liquid crystal display device also is responsive to a datavoltage applied to a liquid crystal cell in the previous display frame.Accordingly, in the conventional LCD, a residual image of the previousdisplay frame remains on the liquid crystal display screen. In otherwords, since a response time of the liquid crystal display exists whenthe current display frame is turned over into the next display frame,data of the previous display frame remains on the display screen asshown in FIG. 2, thereby deteriorating picture quality. This phenomenonpresents a more serious problem in the case of a moving image.

[0007] To overcome this problem, a LCD that allows an image signal to becompensated at every display frame has been disclosed in JapaneseLaid-open Patent Gazette No. 1991-212615. In this LCD device, a modifieddifference signal is calculated on a basis of a difference signalbetween fields for each display frame. Specifically, a modifieddifference signal is determined based on a difference signal betweenadjacent scanning lines and a level of an image signal. Then themodified difference signal is added to the image signal to eliminate aresidual display image that would otherwise emerge upon the liquidcrystal display screen.

[0008] However, since such a LCD device uses a difference signal betweenfields to construct a single image, i.e., a difference signal betweenadjacent scanning lines, the difference signal may distort the imagesignal. Accordingly, a distorted image may result that is different froman initial image on the liquid crystal display screen. Furthermore, inthe conventional LCD, a voltage difference AVp is generated between avoltage Vpxl that is applied to the liquid crystal cell and an effectivevoltage Veff that remains in the liquid crystal cell, thereby causing aflicker phenomenon.

[0009] In FIG. 3, a pixel unit of the conventional LCD device includes agate electrode G electrically connected to a gate line 2, a drainelectrode D electrically connected to a data line 4, and a thin filmtransistor (TFT) 6 electrically connected to a pixel electrode PXL. Thepixel unit further includes a liquid crystal cell 8 and a storagecapacitor Cst disposed between the pixel electrode PXL and a commonelectrode Vcom.

[0010] The TFT 6 is selectively turned on by a pulse-shaped gate highvoltage, as shown in FIG. 4, and electrically connects the data line 4to the liquid crystal cell 8 and the storage capacitor Cst. The liquidcrystal cell 8 and the storage capacitor Cst are charged with a datavoltage VD from the data line 4 when the TFT 6 is turned on, andmaintains the same voltage until the TFT 6 is again turned on (i.e.,when a high voltage Vgh is applied to the gate electrode). When avoltage on the gate electrode is changed from a high voltage Vgh to alow voltage Vgl (i.e., when the TFT 6 is turned off) a voltage VLC atthe liquid crystal cell decreases by ΔVp.

[0011] A voltage difference ΔVp between an effective voltage Veffremaining in the liquid crystal cell and a voltage Vpxl that is appliedto the liquid crystal cell is given by the following equation:

ΔVp=Cgs(Vgh−Vgl)/Cgs+Cst+Clc  (1)

[0012] wherein Cgs represents a parasitic capacitance between the gateand source electrodes, Cst represents a storage capacitor value, Clcrepresents a capacitance of the liquid crystal cell, Vgh represents agate high voltage, and Vgl represents a gate low voltage.

[0013] It can be seen from the above equation (1) that ΔVp is mainlydominated by the parasitic capacitance Cgs and a voltage difference(i.e., Vgh−Vgl) of the gate voltage. In a liquid crystal cell havingpositive and negative data voltages as shown in FIG. 4, an effectivevoltage Veff that remains within the liquid crystal cell becomes lowerthan a voltage Vpxl that is applied to the liquid crystal cell by ΔVp.In particular, since the screen has a different brightness as ΔVpbecomes different for each liquid crystal cell, a flicker phenomenonoccurs. A major reason for this difference in brightness is that acapacitance Clc of the liquid crystal cell becomes different for eachliquid crystal cell due to an affect of the previous data when a newdata is applied.

SUMMARY OF THE INVENTION

[0014] Accordingly, the present invention is directed to a method ofdriving a liquid crystal display that substantially obviates one or moreof the problems due to limitations and disadvantages of the related art.

[0015] An object of the present invention is to provide a method ofdriving a liquid crystal display device to reduce flicker.

[0016] Another object of the present invention is to provide a method ofdriving a liquid crystal display that is capable of preventinggeneration of a residual image and improving picture quality.

[0017] Additional features and advantages of the invention will be setforth in the description which follows, and in part will be apparentfrom the description, or may be learned by practice of the invention.The objective and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

[0018] In order to achieve these and other objects of the invention, amethod of driving a liquid crystal display device during one displayframe includes the steps of applying one of a high-level common voltageand a low-level common voltage to a plurality of liquid crystal cells ofthe liquid crystal display device to write data into the liquid crystalcells within a time interval shorter than one display frame interval,and turning on a backlight after said data writing to display an image.

[0019] In another aspect of the present invention, a method of driving aliquid crystal display device during one display frame includes thesteps of inputting data signals to a plurality of liquid crystal cells,and allowing the liquid crystal cells time to respond to the applieddata signals, wherein one of a high-level common voltage and a low-levelcommon voltage is applied to the liquid crystal cells as a referencevoltage during the inputting step.

[0020] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory and intended to provide further explanation of the inventionas claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings, which are included to provide afurther understanding of the invention and are incorporated in andconstitute a part of this specification, illustrate embodiments of theinvention and together with the description serve to explain theprinciples of the invention. In the drawings:

[0022]FIG. 1 shows a conventional method of driving a liquid crystaldisplay;

[0023]FIG. 2 shows an instantaneous moving image in the conventionalliquid crystal display;

[0024]FIG. 3 is an equivalent circuit diagram of each liquid crystalcell in the conventional liquid crystal display;

[0025]FIG. 4 is a waveform diagram showing voltages applied to theconventional liquid crystal cell shown in FIG. 3;

[0026]FIG. 5 is a diagram for explaining a method of driving a liquidcrystal display according to an embodiment of the present invention;

[0027]FIG. 6 is a waveform diagram representing a variation in a voltageapplied to a liquid crystal cell by the liquid crystal display drivingmethod shown in FIG. 5; and

[0028]FIG. 7 is a graphical representation showing a relationshipbetween a capacitance of the liquid crystal cell and a voltage appliedto the liquid crystal cell in the liquid crystal display driving methodaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] In FIGS. 5 and 6, a display frame is divided into a period atwhich a data is written into a liquid crystal cell (DATA WRITINGPERIOD), a period at which a voltage Vcomh higher or a voltage Vcomllower than a reference common voltage Vcom is applied to the liquidcrystal cell during a response time interval of a liquid crystal whendata is being applied to the liquid crystal cell (COMMON VOLTAGEAPPLICATION PERIOD), a response period of a liquid crystal after thedata is written (LIQUID CRYSTAL RESPONSE PERIOD), and a period at whicha backlight is turned on or off (BACKLIGHT TURNING-ON PERIOD), and aperiod at which a liquid crystal is re-aligned (a second LIQUID CRYSTALRESPONSE PERIOD), for time divisional driving of the LCD.

[0030] In the period at which a data is written into the liquid crystalcell (DATA WRITING PERIOD), one display frame data is recorded into theliquid crystal cell at a shorter time interval than one display frameinterval of 16.67 ms. To this end, as shown in FIG. 6, a high-levelcommon voltage Vcomh higher than, or a low-level common voltage Vcomllower than, a common voltage Vcom typically applied to the conventionalLCD is applied to an upper substrate. In general, a gate high voltageVgh and a gate low voltage Vgl are used as the high-level common voltageVcomh and the low-level common voltage Vcoml, respectively. For example,when a common voltage is set to 5V, the gate high voltage Vgh may be setto 20V and the gate low voltage Vgl to −5V. Thus, the high-level commonvoltage Vcomh is set to more than +15V and the low-level common voltageVcoml is set to less than −5V. Such common voltages Vcomh and Vcoml havevalues substantially larger than or substantially lower than a datavoltage Vpxl that is applied to the liquid crystal cell. Accordingly, asshown in FIG. 6, an effective voltage Veff remaining in the liquidcrystal cell in the data-writing period becomes larger than a voltageVpxl that is applied to the liquid crystal cell.

[0031] If the high-level common voltage Vcomh or the low-level commonvoltage Vcoml is large, as mentioned above, then the effective voltageVeff remaining in the liquid crystal cell can be maintained at a largevalue. Thus, all the liquid crystal cells maintain a black state in anormally white (NW) mode while maintaining a white state in a normallyblack (NB) mode.

[0032] In the response period of the liquid crystal (LIQUID CRYSTALRESPONSE PERIOD), a conventional reference voltage is applied as thecommon voltage Vcom, thereby allowing a real data voltage Vpxl to beapplied to the liquid crystal cell. Thus, the liquid crystal isre-aligned in conformity to the data voltage Vpxl that is applied to theliquid crystal cell. Herein, a time required for such a liquid crystalre-alignment is a liquid crystal response time Tf.

[0033] In the backlight turning-on period (BACKLIGHT TURNING-ON PERIOD)after such a response time Tf of the liquid crystal cell, the backlightis turned on to display an image upon the LCD screen.

[0034] In the liquid crystal re-alignment period (a second LIQUIDCRYSTAL RESPONSE PERIOD), a high-level common voltage Vcomh or alow-level common voltage Vcoml is again applied to the liquid crystalcell as the common voltage Vcom. This allows the polarity of the commonvoltage Vcom that is applied to the liquid crystal cell to be invertedevery frame so as to prevent a deterioration caused by a direct currentvoltage. Accordingly, the liquid crystal is re-aligned in response to aneffective voltage larger than the polarity-inverted data voltage toprepare for the next display frame. A time required for such a liquidcrystal re-alignment is a response time Tr.

[0035] It is desirable that the above-mentioned LCD driving methodemploys an optically compensated bend (OCB) mode or a ferroelectricliquid crystal mode (FLC) that permits a fast driving of the LCD withinone display frame. Alternatively, in a twisted nematic (TN) mode, thehigh-level common voltage Vcomh and the low-level common voltage Vcomlhave a large difference from the conventional common voltage Vcom,thereby allowing a large voltage difference to be generated between aneffective voltage Veff that remains within the liquid crystal cell and avoltage Vpxl that is actualy applied to the liquid crystal cell.Accordingly, since the TN mode permits a fast response speed of theliquid crystal cell, the TN mode also is applicable to the LCD drivingmethod according to the present invention.

[0036] According to the LCD driving method of the present invention, anapplication of the high-level common voltage Vcomh or the low-levelcommon voltage Vcoml permits an almost identical liquid crystalalignment when a data voltage is applied to the liquid crystal cell, sothat the liquid crystal cell always has substantially the samecapacitance value as shown in FIG. 7. More specifically, a high-levelcommon voltage Vcomh or a low-level common voltage Vcoml is applied tothe upper substrate when a data voltage is applied to each liquidcrystal cell, thereby allowing a capacitance Clc of the liquid crystalcell to have an almost equal value regardless of the data voltage. Thus,a difference voltage ΔVp between the effective voltage Veff remaining inthe liquid crystal cell and the voltage Vpxl that is applied to theliquid crystal cell always has substantially the same value.Accordingly, it becomes possible to prevent image quality deteriorationcaused by a flicker phenomenon in the conventional LCD.

[0037] Meanwhile, if an OCB mode is applied to the LCD driving methodaccording to the present invention, then an average applied effectivevoltage always has a larger value than a voltage when an alignment ofthe liquid crystal takes a bend state. As a result, it becomes possibleto prevent a bend state of the liquid crystal, generated when a commonvoltage at the upper substrate is changed, from being returned to asplay state, thereby improving brightness of the display image. Also, analignment film can be made to have a low pre-tilt, so that it becomeseasier to form the alignment film.

[0038] As described above and in accordance with the present invention,the amount of change in the voltage applied in a general liquid crystalmode is increased, thereby permitting a fast response of the liquidcrystal. Accordingly, it becomes possible to improve a contrastefficiency caused by a residual display image. Furthermore, by applyingto the common electrode a voltage higher than or lower than a typicalcommon voltage applied in the conventional LCD, the liquid crystal cellis caused to always have substantially the same capacitance value.Accordingly, a voltage difference between an effective voltage remainingin the liquid crystal cell and a voltage actually applied to the liquidcrystal cell always has substantially the same value regardless of avoltage applied to the liquid crystal cell, thereby preventing theflicker phenomenon.

[0039] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the method of driving aliquid crystal display of the present invention without departing fromthe spirit or scope of the invention. Thus, it is intended that thepresent invention covers the modifications and variations of thisinvention provided they come within the scope of the appended claims andtheir equivalents.

What is claimed is:
 1. A method of driving a liquid crystal displaydevice during one display frame, comprising the steps of: applying oneof a high-level common voltage and a low-level common voltage to aplurality of liquid crystal cells of the liquid crystal display deviceto write data into the liquid crystal cells within a time intervalshorter than one display frame interval; and turning on a backlightafter said data writing to display an image.
 2. The method according toclaim 1, further comprising the step of: allowing the liquid crystalcells to respond according to the data written between the time when thedata is written and when the backlight is turned on.
 3. The methodaccording to claim 2, wherein during the step of allowing, a commonvoltage lower than the high-level common voltage and greater than thelow-level common voltage is applied as a reference voltage to the liquidcrystal cells.
 4. The method according to claim 1, further comprisingthe step of: re-aligning the liquid crystal cells after the step ofturning on the backlight.
 5. The method according to claim 4, wherein atthe step of re-aligning, one of the high-level common voltage or thelow-level common voltage is applied.
 6. The method according to claim 4,wherein at the step of re-aligning, a common voltage having a polarityopposite to the common voltage applied when the data is written isapplied.
 7. The method according to claim 1, wherein when data is beingwritten, an effective voltage remaining in the liquid crystal cell islarger than a data voltage applied to the liquid crystal cell.
 8. Themethod according to claim 1, wherein the high-level common voltage isequal to or more than +15V.
 9. The method according to claim 8, whereinthe high-level common voltage is equal to a gate high voltage applied toa gate electrode of a thin film in transistor of the liquid crystalcell.
 10. The method according to claim 1, wherein the low-level commonvoltage is equal to or less than −5V.
 11. The method according to claim10, wherein the low-level common voltage is equal to a gate low voltageapplied to a gate electrode of a thin film transistor in the liquidcrystal cell.
 12. The method according to claim 1, wherein the drivingmethod is applied to one of an optically compensated bend mode, aferroelectric liquid crystal mode and a twisted nematic mode liquidcrystal display device.
 13. A method of driving a liquid crystal displaydevice during one display frame, the method comprising the steps of:inputting data signals to a plurality of liquid crystal cells; andallowing the liquid crystal cells time to respond to the applied datasignals, wherein one of a high-level common voltage and a low-levelcommon voltage is applied to the liquid crystal cells as a referencevoltage during the inputting step.
 14. The method according to claim 13,wherein a common voltage lower than the high-level common voltage andgreater than the low-level common voltage is applied to the liquidcrystal cells as the reference voltage during the allowing step.
 15. Themethod according to claim 13, further comprising the step of: turning ona backlight after the step of allowing.
 16. The method according toclaim 15, wherein one of the high-level and low-level common voltage isapplied to the liquid crystal cells as the reference voltage during thestep of turning on.
 17. The method according to claim 15, furthercomprising the step of: re-aligning the liquid crystal cells after thestep of turning on.
 18. The method according to claim 17, wherein one ofthe high-level and low-level common voltage is applied to the liquidcrystal cells during the step of re-aligning.
 19. The method accordingto claim 17, wherein during the step of re-aligning, a common voltageapplied to the liquid crystal cells has a polarity opposite to thereference voltage during the step of inputting.
 20. The method accordingto claim 13, wherein the high-level voltage is equal to or more than+15V.
 21. The method according to claim 13, wherein the high-levelcommon voltage is equal to a gate high voltage applied to a gateelectrode of a thin film transistor of the liquid crystal cell.
 22. Themethod according to claim 13, wherein the low-level common voltage isequal to or less than −5V.
 23. The method according to claim 13, whereinthe low-level common voltage is equal to a gate low voltage applied to agate electrode of a thin film transistor in the liquid crystal cell. 24.The method according to claim 13, wherein the driving method is appliedto one of an optically compensated bend mode, a ferroelectric liquidcrystal mode and a twisted nematic mode liquid crystal display device.